Re: [PATCH v3 01/15] stacktrace/x86: add function for detecting reliable stack traces

[Petr Mladek <pmladek@suse.com>]

On Thu 2016-12-08 12:08:26, Josh Poimboeuf wrote:
> For live patching and possibly other use cases, a stack trace is only
> useful if it can be assured that it's completely reliable.  Add a new
> save_stack_trace_tsk_reliable() function to achieve that.
> 
> Scenarios which indicate that a stack trace may be unreliable:
> 
> - running task

It seems that this has to be enforced by save_stack_trace_tsk_reliable()
caller. It should be mentioned in the function description.


> - interrupt stack

I guess that it is detected by saved regs on the stack. And it covers
also dynamic changes like kprobes. Do I get it correctly, please? 

What about ftrace? Is ftrace without regs safe and detected?


> - preemption

I wonder if some very active kthreads might almost always be
preempted using irq in preemptive kernel. Then they block
the conversion with the non-reliable stacks. Have you noticed
such problems, please?


> - corrupted stack data
> - stack grows the wrong way

This is detected in unwind_next_frame() and passed via state->error.
Am I right?


> - stack walk doesn't reach the bottom
> - user didn't provide a large enough entries array
> 
> Also add CONFIG_HAVE_RELIABLE_STACKTRACE so arch-independent code can
> determine at build time whether the function is implemented.
> 
> diff --git a/arch/x86/kernel/stacktrace.c b/arch/x86/kernel/stacktrace.c
> index 0653788..3e0cf5e 100644
> --- a/arch/x86/kernel/stacktrace.c
> +++ b/arch/x86/kernel/stacktrace.c
> @@ -74,6 +74,64 @@ void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
>  }
>  EXPORT_SYMBOL_GPL(save_stack_trace_tsk);
>  
> +#ifdef CONFIG_HAVE_RELIABLE_STACKTRACE
> +static int __save_stack_trace_reliable(struct stack_trace *trace,
> +				       struct task_struct *task)
> +{
> +	struct unwind_state state;
> +	struct pt_regs *regs;
> +	unsigned long addr;
> +
> +	for (unwind_start(&state, task, NULL, NULL); !unwind_done(&state);
> +	     unwind_next_frame(&state)) {
> +
> +		regs = unwind_get_entry_regs(&state);
> +		if (regs) {
> +			/*
> +			 * Preemption and page faults on the stack can make
> +			 * frame pointers unreliable.
> +			 */
> +			if (!user_mode(regs))
> +				return -1;

By other words, it we find regs on the stack, it almost always mean
a non-reliable stack. The only exception is when we are in the
userspace mode. Do I get it correctly, please?

> +
> +			/*
> +			 * This frame contains the (user mode) pt_regs at the
> +			 * end of the stack.  Finish the unwind.
> +			 */
> +			unwind_next_frame(&state);
> +			break;
> +		}
> +
> +		addr = unwind_get_return_address(&state);
> +		if (!addr || save_stack_address(trace, addr, false))
> +			return -1;
> +	}
> +
> +	if (!unwind_done(&state) || unwind_error(&state))
> +		return -1;
> +
> +	if (trace->nr_entries < trace->max_entries)
> +		trace->entries[trace->nr_entries++] = ULONG_MAX;
> +
> +	return 0;
> +}

Great work! I am surprised that it looks so straightforward.

I still have to think and investigate it more. But it looks
very promissing.

Best Regards,
Petr